My research began because of water damage on the front porch roof several months ago. After inspecting it, a roofer said it needed replacing. His estimate was just shy of $3,000, but he warned that any rotten rafters or other structural defects could double the cost.

Because I had been thinking about a green roof for a while, I wanted to compare his estimate with a green roof’s cost. I contacted DC Greenworks, which set up my rain barrel three years ago as part of D.C.’s RiverSmart program, and which also installs green roofs. Although the company’s Web site doesn’t give much information on the installation process (just the many benefits and basic design), I learned a lot through Andrew Benenati, their project manager.

Andrew said the first step is having a structural engineer assess whether the roof can support the extra weight of a green roof system (root barrier, drainage, growing medium and plants). For my small roof (150 square feet), an “extensive” system that uses only three to four inches of a light-weight “growing medium” would probably be best, instead of a deeper, “intensive” system. The question was whether my roof could support it, or if modifications were necessary.

●The roofer to build the new roof deck and install a waterproofing membrane (open);

●An integrity inspection and possible waterproof test (a third-party inspection is especially needed if a roof has not been rebuilt); and

●Installation of the green roof system by DC Greenworks ($20 to $30 per square foot based on the size of my roof).

●A maintenance plan through DC Greenworks is optional.

Also, the District Department of the Environment, through the Anacostia Watershed Society, provides a rebate of $5 per square foot, and DC Greenworks would help with the application and permits.

I wasn’t sure I could afford all this, even with the rebate. However, as everything depended on the structural engineer’s assessment anyway, whether I built now or in the future, I decided to at least get that and the rebuild estimates.

I learned from the results of the structural assessment that a normal roof must support both the weight of the roof and of a person (about 40 pounds per square foot). To add an extensive green roof, a roof must support at least 75 pounds per square foot total. My roof could support this weight only if a new, second set of rafters were bolted (“sistered”) to the existing 2-by-6-inch rafters and if more bolts connected the roof to the house. (A heavier system would have required metal supports.) As my roof needed to be replaced anyway, these modifications would be easy.

I got estimates from three roofers. All said they could do these modifications in addition to the roof, so I did not need a separate structural contractor. Estimates ranged from $4,200 to $5,000, bringing the entire project total to more than $8,500.

At first glance, $8,500 is steep compared to the $3,000 for a traditional roof replacement, but there was a chance a traditional replacement would cost at least $5,000 if the structure had water damage. I was thinking I still couldn’t afford this, but DC Greenworks worked with me to lower the cost. For starters, I will volunteer during the installation to save staff costs, and I’ll do all the maintenance. Add in the potential rebate of $750 for my roof size, and the comparison is now about $7,000 to $5,000.

There are other cost considerations as well. According to DC Greenworks, green roofs can last 30 to 60 years longer than a traditional roof and, in the future, potentially result in discounted D.C. stormwater fees and impervious surface area charges. Plus, it’ll just look better.

The roof at Annette Olson's home in Petworth. (Photo by Annette L. Olson)